Inductive Speed Detector

ABSTRACT

Arrangement for inductive speed detection in which two planar coils are arranged offset at a given distance from a so-called pulse wheel and in the rotational direction of the pulse wheel, and are each part of an oscillator, the oscillator frequency of which is evaluated.

The invention relates to an arrangement for inductive speed detection in accordance with the preamble to patent claim 1. Such arrangements are known for instance from Offenlegungsschrift DE 10 2005 029 764 A1. In a first embodiment thereof the sensor element is embodied as a Hall sensor element. Hall sensor elements are very suitable for detecting variations in a magnetic field and are therefore frequently used for speed detection. Hall sensor elements are inexpensive to acquire and can also be used at temperatures of up to about 160° C.

Hall sensors are disadvantageous in that they are sensitive to dirt particles. In the worst case soiling can lead to the sensors failing entirely.

In accordance with DE 10 2005 029 764 A1, the sensor element can alternatively be embodied as a magneto-resistive (MR) sensor element. The sensor is inductive in another alternative embodiment. In principle inductive sensor elements are also well suited for detecting variations in a magnetic field.

In the known inductive speed detectors, wound coils are used that, in addition to high material expense, are also associated with the definite disadvantage that they are on average several centimeters deep.

The following table lists a few publications that are representative of Hall sensors and inductive sensors that have been used in the past:

Application number Title Patent holder Image Status Pat. Family DE4341405 Arrangement for detecting theposition of rotating shafts AB ElektronikGmbH

Lapsed DE designpatent lapsed DE4445378 Method and device for detecting theangular position of a rotating shaft AB ElektronikGmbh

Lapsed EP; JP; US;WO DE10114858 Crankshaft sensor IC AB ElektronikGmbH

Lapsed DE designpatentregisteredand pending DE2847522 Inductive transducer and evaluationcircuit for same Robert BoschGmbH

Lapsed JP; FR DE2939643 Inductive speed or angle of rotationtransducer Robert BoschGmbH

Lapsed GB; FR; IT DE3242109 Device for detecting the speed of arotating part Robert BoschGmbH

DE Lapsed EP; JP; USall had tolapse DE4431640 Sensor arrangement for determiningthe speed of a moving component Robert BoschGmbH

Lapsed —

Known from the pamphlet “Digital Inductive Position, Speed, and Direction Sensor PO120”, pages 1 through 3, is a digital inductive sensor for linear position determination, angle measurement, and speed measurement having tooth-shaped pulse wheels. The measurement coils are flat and resistant in oil, water, dust, and in magnetic fields. According to the transformer principle, the sensor chip includes a transmitter coil and two receiver coils. The receiver coils are offset in the direction of movement of the pulse wheel (of the target). The possible distance between target and sensor chip is also limited in this arrangement.

The object of the invention is to overcome the problems of the prior art, i.e. to provide an arrangement for inductive speed detection that is not sensitive with respect to soiling by particles, that enables shallow installation depth, and that permits a great distance to the generator.

The tacho generator should also withstand high temperatures and high pressures such as e.g. in a transmission.

In accordance with claim 1, the basic solution is comprised in the use of two planar coils for measuring speed. The two coils are arranged such that they are spatially offset in the direction of rotation. Detection by means of the coils occurs via a moved component, a so-called pulse wheel. The prerequisite for this pulse wheel is that it can periodically influence the inductive sensor. The structure of the pulse wheel can either resemble that of a toothed wheel (see FIG. 1) or it can be constructed like a lattice (see schematic depiction in FIG. 2). The pulse wheel does not have to be coded to be able to detect the direction in which it rotates. What is determinant for this is that the two planar coils are arranged spatially offset in the direction of rotation. FIG. 3 depicts this spatial offset for the two flat coils. The coil 1 can be a part of an oscillator and the coil 2 can also be part of another oscillator, in each of which the oscillator frequencies are determined. Frequency evaluation is more precise than conventional voltage or current evaluation due to higher resolution.

If, in a special case, the issue does not involve detecting the direction of rotation, a single planar coil can also be adequate for determining speed, as provided in coordinate claim 12. All of the advantages that accrue from the oscillation frequency evaluation of the associated oscillator also apply in this case.

A large effective coil surface area is obtained using the special arrangement of the coils 1 and 2 in FIG. 3 (or corresponding to a single planar coil). This is important in order to also be able to undertake precise evaluations even when there is a great distance to the pulse wheel.

The coil 1 (and the coil 2, if any) are preferably printed on a printed circuit board, but can also be stamped or can comprise curved wire, etc.

In order to prevent the two coils 1 and 2 from influencing one another, they are switched through a multiplexer such that at all times current is flowing through only one coil and the other coil is highly resistive during this period. In the inventive speed sensor the evaluation of frequency changes strongly contributes to supporting detection, even given great distances. Detection under these conditions represents a substantial advantage. With known arrangements in the past it has only been possible to attain a distance of no more than 4 to 5 mm between sensor and pulse wheel. With the inventive arrangement it is even possible to work at a distance of up to 10 mm.

Thanks to the great distance, the pulse generator (pulse wheel) can be exchanged as desired within an application. In the prior art there was the problem that when using smaller generator wheels (pulse wheels) the sensor did not provide any more usable results due to the greater distance. Combining planar coils and frequency evaluation counteracts this disadvantage.

Rotational direction detection is also provided in a particularly simple manner when the advancing of a tooth or lattice element (at least the passing of a pulse-originated segment of the wheel circumference) is first perceived as a pulse on one flat coil and then on the other flat coil. In FIG. 4 it is possible to see how the sequence of the pulse flanks reverses with the direction of rotation.

In the second solution in accordance with claims 12 through 21, this detection of the direction of rotation is not provided with nothing further, but can be added in a manner alternative to the first solution, specifically if the pulse wheel is coded in the sense of claim 13. Coding means that the individual teeth of the pulse wheel can be differentiated. Then use for detecting the direction of rotation in accordance with claim 22 is possible.

The following additional technical features apply for both solutions:

-   -   The pulse wheel does not have to be a magnetic actuator, but         rather can be an electrically conductive eddy current damping         element. If coded, each tooth or each interval between teeth for         instance has different conductivity.     -   The so-called pulse wheel can resemble a toothed wheel or it can         be constructed on its circumference like a periodic lattice.     -   It is a great advantage that the pulse wheel can be exchanged         for a different, non-identical pulse wheel because the         importance of the spacing in the detection arrangement was         reduced. The permissible assembly tolerances are therefore         greater. The distance between planar coil and pulse wheel can be         up to 10 mm.     -   The planar coils are printed on a printed circuit board, perhaps         using multilayer engineering. However, flat coils can also         alternatively be produced by punching from sheet or by bending         wire.     -   For detecting speed, the absolute oscillation frequency is not         evaluated (the base frequencies are in the MHz range, for         instance about 20 MHz or about 400 MHz), but rather the changes         in frequency when a pulse tooth passes by are detected. An         example shall explain this in closing.

At room temperature (20 degrees Celsius), let the base frequency be 20.0 MHz. The pulse temporarily increases this oscillation frequency to 20.2 MHz; i.e., the pulse causes a short-term 0.2 MHz change in frequency (which can be detected easily and can be associated with one tooth).

Although the same experiment at a transmission temperature of 150 degrees Celsius results in an increased base frequency of 24.5 MHz, it provides a constant 0.2 MHz change in frequency to 24.7 MHz now. Thus, using the invention it is possible to associate a constant change in frequency (i.e. a temperature-stable and distance-compensatable change in frequency) to each pulse-initiating element. Such pulses can be easily and reliably counted in the evaluation logic element.

When the pulse wheel is coded, the frequency-increasing pulses in the same sense are constant (that is, temperature-stable and distance-compensatable); they differ as well only from tooth to tooth in step shape or in their temporal change. The evaluation circuit can then also localize each individual tooth successively, i.e. can detect the rotational direction and/or the rough angular position from this individualization. 

1. Arrangement for inductive speed and rotational direction detection in which two planar coils (1, 2) are arranged offset at a given distance from a so-called pulse wheel and in the rotational direction of said pulse wheel, and are each part of an oscillator, the oscillator frequency of which is evaluated.
 2. Arrangement in accordance with claim 1, characterized in that said pulse wheel is not a magnetic actuator.
 3. Arrangement in accordance with claim 1 or 2, said pulse wheel being constructed periodically like a lattice.
 4. Arrangement in accordance with claim 1 or 2, said pulse wheel resembling a toothed wheel.
 5. Arrangement in accordance with claims 1 or 2, characterized in that said pulse wheel is exchangeable.
 6. Arrangement in accordance with claims 1 or 2, characterized by a distance between said planer coils (1, 2) and said pulse wheel of up to about 10 mm.
 7. Arrangement in accordance with claims 1 or 2, characterized in that said planar coils (1, 2) are printed on a printed circuit board, possibly using multilayer engineering.
 8. Arrangement in accordance with claims 1 or 2, characterized in that said planar coils (1, 2) are punched or bent wire.
 9. Arrangement in accordance with claims 1 or 2, characterized in that the oscillation frequencies of said oscillators are in the range of a base frequency of about 20 to about 400 MHz.
 10. Arrangement in accordance with claims 1 or 2, characterized in that said planar coils (1, 2) are switched to said oscillator via a multiplexer.
 11. Arrangement for inductive speed and rotational direction detection in which said one planar coil (1) is arranged at a distance from a so-called pulse wheel and said pulse wheel is coded and said planar coil (1) is part of an oscillator, the oscillator frequency of which is evaluated.
 12. Arrangement in accordance with claim 11, characterized in that said pulse wheel is not a magnetic actuator but rather is electrically conductive.
 13. Arrangement in accordance with claims 11 or 12, said pulse wheel being constructed periodically like a lattice.
 14. Arrangement in accordance with claims 11 or 12, said pulse wheel resembling a toothed wheel.
 15. Arrangement in accordance with claims 11 or 12, characterized in that said pulse wheel is exchangeable.
 16. Arrangement in accordance with claims 11 or 12, characterized by a distance between said planer coil (1) and said pulse wheel of up to about 10 mm.
 17. Arrangement in accordance with claims 11 or 12, characterized in that said planar coil (1) is printed on a printed circuit board.
 18. Arrangement in accordance with claims 11 or 12, characterized in that said planar coil (1) is punched or bent wire.
 19. Arrangement in accordance with claims 11 or 12, characterized in that the oscillation frequency of said oscillator is in the range of a base frequency of about 20 to about 400 MHz. 